Field of the Invention
The present invention relates to a method of handling at least a call in handover in a wireless communication system and a wireless communication device using the same, and more particularly, to a method of handling at least a packet-switched (PS) call handover to at least a circuit-switched (CS) call in different wireless communication networks and a wireless communication device using the same.
Description of Related Art
Currently, evolved Universal Telephone System Terrestrial Radio Access Network (E-UTRAN) is designed to have only packet-switched (PS) connections, and voice call applications in the E-UTRAN have to be provided in a form of voice over Internet Protocol (VoIP) application or an IP multimedia subsystem (IMS) voice call. However, most operators running the E-UTRAN may still have other types of networks based on other wireless communication technologies, and the operators would like to have a seamless and smooth handover of the VoIP call or the IMS voice call from the E-UTRAN to other networks. For example, one of the operators running the E-UTRAN may have an UTRAN and a Global System for Mobile communications (GSM) Enhanced data rate for GSM evolution radio access network (thereafter abbreviated as GERAN). When a user equipment (UE) with multicall capability has the IMS voice call under one of the PS domain connections in the E-UTRAN and the E-UTRAN initiates a single radio voice call continuity (SRVCC) handover correspondingly, the UE may transfer the IMS voice call to a CS call in either the UTRAN or the GERAN since the CS call has better quality of service (QoS) capabilities such as low latency, low bit error rate, and low timing jitter.
It is currently not clear on whether the UE and the UTRAN (or the GERAN) sets the identical stream identifier (SI) for the CS call. Usually, the UE supporting multicall (i.e. having multicall capability) includes the SI in a SETUP message which is to be sent to the UTRAN or the GERAN. The networks (e.g., the UTRAN or the GERAN) which support multicall may interpret the SI value after receiving the SETUP message from the UE as the following. If the UE generates a new SI value at an initiation of an originating call, then a new traffic channel has to be assigned to the originating call. On the contrary, if the UE just indicates an existing SI value, then the indicated traffic channel has to be used for the originating call. The UE supporting multicall shall never send an additional SETUP with indication that a new traffic channel is requested to a wireless communication network that does not support multicall.
Moreover, if the UE supporting the multicall, the UE has to include the SI information element in the SETUP message. For the first call (i.e., when there is no other ongoing call), the SI may initially start from 1. Usually the SI value may have three bits. However, if the UE does not set the SI value for the CS call but the network sets it to 1, later on when the UE initiates a CS streaming call and the UE sets the SI value to 1 for the CS streaming call, it may cause the streaming CS call setup failed since the UTRAN (or the GERAN) considers the SI=1 already been set for one CS voice call. Beside, the UTRAN or the GERAN cannot use the traffic channel configuration of the CS voice call for the CS streaming call since radio access bearer (RAB) identifier or the traffic channel identifier is required to be identical to the SI. In addition, it is also currently not clear how to handle multicall in handover from the E-UTRAN to either the UTRAN or the GERAN.
Therefore, how to handle calls or multicall in handover in wireless communication systems and related communication devices becomes an important issue to be researched and discussed.